Provided is an air pollution control system including: a denitration apparatus 12 that is configured to remove nitrogen oxides in a flue gas 18 discharged from a boiler 11; an air heater 13 that is configured to recover heat of the flue gas 18 after denitration; a precipitator 14 that is configured to remove soot and dust contained in the flue gas after heat recovery in the form of collected dust ash 16; a desulfurization apparatus 15 that is configured to remove sulfur oxides contained in the flue gas 18 after dust removal using limestone slurry 20 which is an absorbent; a dehydrator 32 that is configured to remove gypsum 31 from absorber slurry discharged from the desulfurization apparatus 15; a spray drying apparatus 50 provided with a spray unit that is configured to spray dehydrated filtrate 33 as desulfurization wastewater supplied from the dehydrator 32; a flue gas introduction line L 11 through which a branch gas 18a branched from the flue gas 18 is introduced to the spray drying apparatus 50; a flue gas supply line L 12 through which flue gas 18b returns to a main flue gas duct, the flue gas 18b being obtained after the dehydrated filtrate is dried by the spray drying apparatus; and a powder supply apparatus 60 that is configured to supply a powder 61 to the flue gas introduction line L 11 .